It appears that Winamp's RG is meant for stereo. Have you enabled the "Allow surround sound" option in the General Preferences' Playback tab?
Even so, there are various multichannel channel configurations and I'm not sure what Winamp supports (it probably just passes thru what's in the source file) and if the configurations match what the computer's sound device hardware supports. See the "Standard speaker channels" chart in the linked wikipedia article (
https://en.wikipedia.org/wiki/Surrou...surround_sound). This is why I normally use music source files that only have 1 or 2 channels; along with the WinampMatrixMixer + DirectSound plug-ins to specify how much of each content channel is played through each speaker channel in order to match up with my hardware and OS settings. It is even more complicated to get video files with multichannel sound tracks to play correctly in Winamp, so I usually use a different app for those. Even then, I can't get 'correct' playback if the video file has a different configuration and/or number of channels than what my 5.1 speaker system supports.